1. Field of the Invention
The present invention relates to a nonlinear dielectric element, for example, a capacitor for generating high voltage pulses used for a high-intensity discharge lamp, which exhibits a characteristic of nonlinear behavior in an electric field-electric charge. In particular, the present invention relates to a nonlinear dielectric element in which insulation layers are formed covering the end parts of electrodes in order to improve insulation performance.
2. Description of the Related Art
High-intensity discharge lamps (HID lamps) are known. Among the HID lamps, there are lamps, such as high-pressure sodium lamps and metal halide lamps, in which high voltage pulses of about 1 to 4 kV are necessary during startup. Therefore, capacitors exhibiting nonlinear characteristics are incorporated in the HID lamps of this nature in order to generate high voltage pulses.
For example, a high-pressure discharge lamp including a nonlinear capacitor for generating high voltage pulses is disclosed in Japanese Examined Patent Application Publication No. 5-87940. The structure of the nonlinear capacitor described in this publication will be explained with reference to FIG. 1.
A nonlinear capacitor 1 has a structure in which electrodes 3 and 4 are formed on both faces of a ceramic board 2 made of barium titanate ceramic. Lead terminals 7 and 8 are connected to each of the central portion of the electrodes 3 and 4, respectively, with junction members 5 and 6 therebetween. The whole structure including the ceramic board 2 and the electrodes 3 and 4, except for the portion where the lead terminals 7 and 8 are lead out, is covered with an insulation layer 9.
The aforementioned electrodes 3 and 4 are formed by applying a conductive paste containing, for example, a silver powder and at least one of borosilicate glass and lead borosilicate glass on the ceramic board 2 by coating, and thereafter, by baking.
The insulation layer 9 is formed by applying a paste containing a glass powder on each of the surfaces of the ceramic board 2 and the electrodes 3 and 4 by coating, and thereafter, by baking.
According to the aforementioned nonlinear capacitor 1, the insulation performance between the electrodes 3 and 4 is improved by being provided with the insulation layer 9, so that a structure suitable for generating high voltage pulses can be achieved.
However, the aforementioned conventional nonlinear capacitor 1 may encounter a problem in that the intrinsic nonlinear characteristics of a ceramic board 2 made of barium titanate ceramic cannot be sufficiently exhibited. This is believed to be because the glass component in the conductive paste used for forming the electrodes 3 and 4 diffuses into the ceramic board 2 during baking, so as to degrade the nonlinear characteristics of the ceramic board 2.
In order to solve the aforementioned problems, it is described in Japanese Unexamined Patent Application Publication No. 11-214257 that when the glass component contained in the electrodes 3 and 4 is made to be at least one of bismuth-containing lead borosilicate glass and barium oxide borosilicate glass, the nonlinear characteristics of the dielectric ceramic constituting the ceramic board 2 can be sufficiently exhibited, and consequently, a nonlinear dielectric element which can exhibit excellent nonlinear characteristics can be realized.
The aforementioned measure relates to an avoidance of a reverse effect on the ceramic board 2 due to the glass component contained in the electrodes 3 and 4. It is a fact that the glass component contained in the electrodes 3 and 4 can be prevented from hindering the nonlinear characteristics of the ceramic board 2.
However, the nonlinear capacitor 1 as shown in FIG. 1, provided with the insulation layer 9, the glass component contained in the insulation layer 9 causes a problem similar to that in the above description. That is, there is a problem in that the glass component contained in the insulation layer 9 is also likely to diffuse into the ceramic board 2, so as to penetrate into grain boundaries of the ceramic constituting the ceramic board 2 and to degrade the nonlinear characteristics thereof. If the nonlinear characteristics are thus degraded, when high voltage pulses are generated by the nonlinear capacitor 1, pulse voltages are reduced.
Accordingly, it is an object of the present invention to provide a nonlinear dielectric element which can solve the aforementioned problems.
According to an aspect of the present invention, a nonlinear dielectric element exhibiting nonlinear behavior in an electric field-electric charge has an element main body composed of a dielectric ceramic exhibiting the nonlinear behavior, first and second electrodes formed on first and second faces opposing each other, respectively, of the element main body, and insulation layers formed on the element main body so as to cover at least a part of the first and second electrodes. Wherein, in order to solve the aforementioned technical problems, the first and second electrodes contain a conductive component and barium oxide borosilicate glass, and the insulation layers contain barium oxide borosilicate glass.
In the present invention, the barium oxide borosilicate glass contained in the insulation layers preferably has the same composition as that of the barium oxide borosilicate glass contained in the first and second electrodes.
In the present invention, the first and second electrodes are preferably formed leaving each of the outer regions of the first and second faces of the element main body uncovered.
In the aforementioned case, the insulation layers are more preferably formed so as to cover from the neighborhood of the rims of the first and second electrodes to the outer regions of the first and second faces.
The nonlinear dielectric element according to the present invention is preferably used as a capacitor for generating high voltage pulses.